Spread spectrum communications is being used for a number of commercial applications and is proliferating at a rapid rate. Synchronous orthogonal direct sequence code division multiple access (ODS-CDMA) has been proposed (see U.S. Pat. No. 5,375,140, xe2x80x9cWireless Direct Sequence Spread Spectrum Digital Cellular Telephone Systemxe2x80x9d, incorporated herein by reference) as an effective technique for improving the capacity, i.e., bandwidth efficiency, of the more conventional quasi-orthogonal CDMA.
In conventional direct sequence (DS) spread spectrum CDMA systems, the individual users transmit on the same frequency using different pseudo-noise (PN) codes. The PN codes are quasi-orthogonal, i.e. they have relatively low but nonzero cross-correlation values with each other.
ODS-CDMA systems are designed such that all signals are received in time and frequency synchronism. Thus all users remain orthogonal to each other and, in an ideal world, any user can be recovered with no multiple access noise from other users. This is most practical in a star configured network where a multiplicity of users transmit to and receive from a single Hub Station. This configuration is used in cellular as well as satellite networks.
In an ODS-CDMA system, each user is assigned a code which is orthogonal to all of the other user codes (i.e. the orthogonal codes have a cross-correlation value of zero with each other). Further, the orthogonal code period is chosen such that the code repeats an integer number of times in a data symbol time (usually once, since this results in the maximum number of available orthogonal functions). The code epoch is synchronized with the symbol transitions so that no data transitions occur within the code. The number of users is limited by the number of orthogonal codes available, which is equal, at most, to the length of the code. Therefore, the chipping rate is equal to the maximum number of orthogonal users times the symbol rate.
Efficient use of the available bandwidth is accomplished by using bi-phase spreading and MPSK data modulation as taught in U.S. Pat. No. 5,687,166, xe2x80x9cModulation System for Spread Spectrum CDMA Communication,xe2x80x9d incorporated herein by reference.
It is often desirable to accommodate a mix of different rate users where the rates are related by 2n where n is a positive integer. One code-efficient way to do this is to size the system for the lowest user rate and then demultiplex higher rate data onto multiple orthogonal codes, i.e. data of a user at 4 times the nominal rate would be demultiplexed and spread onto 4 codes which are then summed for transmission. This scheme, while efficient in the use of orthogonal codes, produces a signal with a wide dynamic range which is a disadvantage for the subscriber terminal power amplifier efficiency. It also requires multiple correlators and multiplexing to recover the original data stream. This technique is discussed by Ejzak, et al in xe2x80x9cBALI: A Solution for High-Speed CDMA Data,xe2x80x9d in Bell Labs Technical Journal, vol. 2, no. 3, Summer 1997.
Another technique is disclosed in U.S. patent application Ser. No. 08/352,313, now U.S. Pat. No. 5,574,721, entitled xe2x80x9cOrthogonal Code Division Multiple Access Communication System Having Multicarrier Modulationxe2x80x9d incorporated herein by reference. In this disclosure, it is suggested that multiple ODS-CDMA signals be transmitted on orthogonally spaced carriers (i.e. spaced at the chipping rate) and the data from a single high rate user is demultiplexed onto the multiple carriers. Once again, this results in a signal with wide amplitude variation.
In U.S. Pat. No. 5,416,797 xe2x80x9cSystem and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System,xe2x80x9d incorporated herein by reference, lower data rates than the nominal are supported on the cell-to-mobile link (which is orthogonal), by repeating data symbols of a low rate user a number of times to obtain the nominal symbol rate. This sequence of symbols is then spread by an orthogonal code and transmitted at a lower power proportional to the lower rate. This technique has the disadvantage of not being orthogonal code space efficient, i.e. each low rate user uses the same amount of code space as a high rate user.
In xe2x80x9cDesign Study for a CDMA-Based Third Generation Mobile Radio System,xe2x80x9d IEEE Journal on Selected Areas in Communication, vol. 12, no. 4, May 1994, Baier, et al, propose to use variable length spreading codes to support variable data rates in a CDMA system. However, they propose to use PN codes that are not necessarily orthogonal.
An object of this invention is to provide a means by which an ODS-CDMA communications system can function efficiently with data rates that are not all equal. This non-homogeneity in data rates allows different users to communicate with different data bandwidths while the ODS-CDMA communications system signaling rate and orthogonal nature remain the same as in a homogeneous data rate system.
A further object of this invention is to provide an ODS-CDMA system which supports a mix of users, with data rates related by 2n, which makes efficient use of the available orthogonal code space, i.e. a user with symbol rate R/k uses 1/k of the code space of a user with symbol rate R.
It is a further object of this invention to ensure that users of all rates transmit a relatively constant amplitude signal. This is of particular importance to the subscriber terminal where high transmitter power efficiency (at or near saturation) is desirable due to the impact on cost and power consumption.
It is a further object of this invention to keep signal processing complexity to a minimum.
In an ODS-CDMA system, each user is assigned a code which is orthogonal to all of the other user codes (i.e. the orthogonal codes have a cross-correlation value of zero with each other). Further, the orthogonal code period is chosen such that the code repeats an integer number of times in a data symbol time (usually once, since this results in the maximum number of available orthogonal functions) and the code epoch is synchronized with the symbol transitions so that no data transitions occur within the code. Thus, in a perfectly synchronized system, the individual users can be demodulated with no interference from other users.
The number of users is limited by the number of orthogonal codes available, which is equal, at most, to the length of the code. Note that the chipping rate is equal to the maximum number of orthogonal users times the symbol rate. This implies that, for a fixed chipping rate, a system transmitting data at a rate of R/4 should be able to accommodate 4 times as many users as a system with rate R.
Efficient use of code space in a system with a mix of user rates ensures that a user with rate R/4 will only occupy 1/4 of the orthogonal function space of a user with rate R. The invention disclosed herein teaches an efficient way to construct such orthogonal codes by selection of an orthogonal codebook wherein subsequences of the codewords are also orthogonal. With such a codebook, short codes for high rate users are determined directly by the subsequences, and long codes for low rate users are determined by the Kronecker product of the subsequences with an orthogonal codeword allocated to these users. As a preferred embodiment, the Sylvester construction of a Hadamard matrix is used to generate the orthogonal codebook. Thus, this invention discloses how to efficiently support variable data rates in a synchronous ODS-CDMA communication system with constant chip rate.
The novelty of this invention lies in the selection of an orthogonal codebook which enables high rate data to be spread using shorter codewords, and conversely, low rate data to be spread using longer codewords, all of which remain mutually orthogonal.
Further, using this invention, each user transmits a single code that is constant in amplitude and requires no data multiplexing or demultiplexing. The disclosed technique results in low receiver and transmitter complexity since no additional code generators or correlators are required to accommodate rates that are higher or lower than the nominal data rate. Further, the constant amplitude nature of the signal allows efficient power amplification.